The magnetic fields produced by current flow in a target well are an extremely valuable tool in the guidance of drilling equipment, and a considerable amount of effort has been devoted to the development of techniques for producing such fields and to the development of highly sensitive equipment for accurately detecting them. The detection equipment may be located in a borehole being drilled, for example, to detect the distance and direction to an existing target well. This information is used to control the direction of drilling so that the borehole can be positioned with respect to the target to intercept it at a desired location, to avoid it, or to pass near it, as may be desired.
Such directional control systems may be used in drilling a new well in a field of existing wells where the existing wells are to be avoided, in drilling a rescue well to intersect an existing well which has blown out, in drilling horizontal collection boreholes adjacent existing wells, and in numerous other applications. For example, a producing oil field typically includes a large number of wells leading generally vertically from the surface of the earth downwardly into oil-bearing strata from which the crude oil is extracted. These wells often are quite close together, particularly when the wells originate at an offshore drilling platform and, as pointed out in U.S. Pat. No. 4,593,770 to Hoehn, the drilling of additional vertical wells within a field requires careful control of the drilling in order to avoid intersection with existing wells. In accordance with the Hoehn patent, such undesired intersections are avoided by lowering, as by means of a support cable, a wireline carrying a bore hole tool into each of the existing wells and injecting into the casings of such existing wells alternating currents which produce corresponding magnetic fields surrounding the existing well casings. A bore hole tool carries contact pads which incorporate electrodes for contacting the casing of the well in which the bore hole tool is located, so that current can flow downhole through the support cable and through the electrodes into the casing. In the Hoehn patent, each of the existing wells is injected with current of a different frequency so that specific wells can be identified by the frequency of their corresponding magnetic fields. A magnetometer in a non-magnetic section of a well being drilled can then measure the magnetic fields produced by the current flow in existing wells during drilling so that the well being drilled can be redirected as required. By the technique described in the '770 patent, a large number of wells can be drilled into an oil bearing field so as to maximize its production.
As a further example, when the oil contained in an oil-bearing field is gradually depleted by the producing wells, the flow of oil to the wells gradually slows, and eventually stops. Often, however, there remains a considerable amount of oil in the strata from which the oil is being drawn, even though the wells have stopped producing, and this remaining oil can be recovered by means of a "rescue" well which is drilled from the surface downwardly to the oil bearing strata. This rescue well is drilled vertically and is then curved to a horizontal attitude and must, in many cases, pass vertically near one or more existing vertical wells without inadvertently intersecting them, and then must pass horizontally through the well field, again without intersecting the producer wells. The horizontal run of the rescue well is guided not only to avoid intersection with the producer wells, but also to pass within about 2 meters of a selected target vertical producer well. The horizontal well passes the target producer and travels beyond it a predetermined distance, and is then sealed at its far end. The horizontal run is then perforated by a multiplicity of holes spaced along its length from its far, or terminal, end toward a near location which is a distance on the near side of the vertical producer well approximately equal to the distance of the far end from the producer well. After perforation, the horizontal section is sealed off at the near location to form a closed near end. This leaves a sealed-off, perforated, intermediate section which forms a right angle, or T, with respect to the target vertical producer well. This perforated section preferably is symmetrical with respect to the target well, and serves to collect oil from the oil bearing strata in the region of the target vertical producer well and to drain that collected oil toward the producer well.
When a system of collectors is to be provided, the rescue well is redrilled above the near-end plug and is again directed horizontally toward a second target producer well in the field. The horizontal rescue well is again drilled to pass near, but to avoid a direct intersection with, the second target vertical producer well and extends past that vertical well by a selected distance. The rescue well is again sealed at its far end, is perforated, and is sealed at a near location which is equidistant from the target vertical well to form a near end, thereby producing a second field-draining intermediate collector section which directs oil to the second target producer well. The rescue well may again be redrilled from the region of the near-end plug and the process repeated for a third and for subsequent target vertical producer wells.
Numerous other applications of borehole drilling techniques which require accurate control of the drilling of a borehole, or rescue well, through a field of existing target wells are known. In each it is critical to the success of the technique that reliable information about the relative locations of the rescue and target wells be available at the earliest possible time during the drilling.
A convenient directional control system for situations where the target wells are open; i.e., where access to the wells is available from the surface, is illustrated in the above-mentioned U.S. Pat. No. 4,593,770. In accordance with that patent, a wireline is dropped down a cased well, and its electrodes contact the casing at a selected depth to inject current. The point within each existing well at which current is injected is a point that is as close as possible to the likely intersection point between the existing well and the well being drilled. Current then flows from that point of injection both upwardly and downwardly in the casing to produce a resultant magnetic field in the earth surrounding the existing well. Because the point of current injection is selected to be at the projected point of intersection of the existing well and the well being drilled, the current flowing down the wireline divides after it is injected into the casing with one half flowing downwardly from the injection point, and one half flowing upwardly from that point toward the surface. The magnetic field produced by the upwardly flowing current in the casing surrounding the wireline is in direct opposition to and is equal to one-half of, the magnetic field produced by the downwardly flowing wireline current. As a result, the net magnetic field above the injection point is one half the magnetic field produced by the wireline current. The magnetic field below the injection point is also reduced by one half that of the wireline current, since only one half of the available current flows downwardly in the casing from that point. Accordingly, using this technique, only one half of the potentially available magnetic field is actually available for use in guiding the well being drilled.